Coded light refers to techniques whereby a signal is embedded in the visible light emitted by a luminaire. The light thus comprises both a visible illumination contribution for illuminating a target environment such as room (typically the primary purpose of the light), and an embedded signal for providing information into the environment. To do this, the light is modulated at a certain modulation frequency or frequencies.
In some of the simplest cases, the signal may comprise a single waveform or even a single tone modulated into the light from a given luminaire. The light emitted by each of a plurality of luminaires may be modulated with a different respective modulation frequency that is unique amongst those luminaires, and the modulation frequency can then serve as an identifier of the luminaire or its light. For example this can be used in a commissioning phase to identify the contribution from each luminaire, or during operation can be used to identify a luminaire in order to control it. In another example, the identification can be used for navigation or other location-based functionality, by mapping the identifier to a known location of a luminaire or information associated with the location.
In other cases, a signal comprising more complex data may embedded in the light. For example using frequency keying, a given luminaire is operable to emit on two (or more) different modulation frequencies and to transmit data bits (or more generally symbols) by switching between the different modulation frequencies. If there are multiple such luminaires emitting in the same environment, each may be arranged to use a different respective plurality of frequencies to perform its respective keying.
WO2012/127439 discloses a technique whereby coded light can be detected using a normal “rolling shutter” type camera, as is often integrated into a mobile device like a mobile phone or tablet. In a rolling-shutter camera, the camera's image capture element is divided into a plurality of lines (typically horizontal lines, i.e. rows) which are exposed in sequence line-by-line. That is, to capture a given frame, first one line is exposed to the light in the target environment, then the next line in the sequence is exposed at a slightly later time, and so forth. Typically the sequence “rolls” in order across the frame, e.g. in rows top to bottom, hence the name “rolling shutter”. When used to capture coded light, this means different lines within a frame capture the light at different times and therefore, if the line rate is high enough relative to the modulation frequency, at different phases of the modulation waveform. Thus the modulation in the light can be detected.